问为什么lua luaZ_fill会这样读取数据

EN

struct Zio {
  size_t n;         /* bytes still unread */
  const char *p;        /* current position in buffer */
  lua_Reader reader;
  void* data;           /* additional data */
  lua_State *L;         /* Lua state (for reader) */
};
int luaZ_fill (ZIO *z) {
  ....
  z->n = size - 1; 
  z->p = buff;
  return char2int(*(z->p++));
}
int luaZ_lookahead (ZIO *z) {
  if (z->n == 0) {
    if (luaZ_fill(z) == EOZ)
      return EOZ;
    else {
      z->n++;  /* luaZ_fill removed first byte; put back it */
      z->p--;
    }
  }
  return char2int(*z->p);
}

int luaZ_fill (ZIO *z) {
  ....
  z->n = size; 
  z->p = buff;
  return char2int(*(z->p));
}
int luaZ_lookahead (ZIO *z) {
  if (z->n == 0) {
    if (luaZ_fill(z) == EOZ)
      return EOZ;
  }
  return char2int(*z->p);
}